Interference is one of the biggest challenges to obtaining high data transmission rates in Code Division Multiple Access (CDMA) systems, such as Wideband CDMA and cdma2000. Interference arises in two main ways. First, in dispersive channels, multiple echoes of the transmitted signal arrive at the receiver with different delays. Such dispersion results in Intersymbol Interference (ISI) and Multiple Access Interference (MAI) due to loss of orthogonality between different spreading codes. Second, neighboring cells contribute interference even in non-dispersive channels because the base station long scrambling codes are not orthogonal. In addition to interference, a receiver must contend with noise. The combination of interference and noise, referred to herein as receiver impairment or impairment, degrades performance at the receiver and limits the data rates that can be realized.
Generalized RAKE (G-RAKE) receivers have been developed for suppressing interference and noise. Interference suppression is achieved in a G-RAKE receiver by treating ISI and MAI as colored Gaussian noise. The noise correlation across fingers is then exploited by adapting the finger delays and combining weights. In this way, the orthogonality between user signals may be partially restored. Recently, further improvements in G-RAKE receivers have been proposed for the High Speed Downlink Packet Access (HSDPA) mode of WCDMA that take into account code cross-correlations.
Multi-user detection techniques have also been used to suppress MAI and ISI due to channel dispersion. Various types of multi-user detectors are known. The optimal multi-user detector is a Maximum Likelihood Sequence Estimation (MLSE) detector. However, the complexity of an MLSE detector grows exponentially with the number of users and is therefore not practical to implement. Therefore, there is interest in developing suboptimal detectors that obtain good performance with low complexity.
One suboptimal multi-user detector is the linear Minimum Mean Squared Error (MMSE) detector. Co-pending application Ser. No. 11/739,126 filed 24 Apr. 2007 and titled “Robust Multicode Detector for HSDPA” describes an exemplary MMSE detector for generating MMSE estimates of the received symbols. For proper decoding, it is desirable to scale the MMSE estimates or other initial symbol estimates of the received symbols to yield maximum likelihood (ML) estimates.